Honeycomb filters are in use in, for example, a filter for capturing fine particles present in exhaust gas emitted from an internal combustion engine, a boiler or the like, particularly, particulate matter in exhaust gas emitted from diesel engine.
The honeycomb filter used for such a purpose generally has, as shown in FIGS. 10(a) and 10(b), a structure having a large number of through-holes (3) extending in an X axis direction divided from each other by partition walls (2), and through-holes (3) are alternately plugged at each end face so that each end face looks checkerboard pattern. In such a honeycomb filter, a subject fluid flows in the through-holes (3) not plugged at inflow end face (42), that is, plugged at outflow end face (44), passes through the porous partition walls (2), and is discharged via the adjacent through-holes (3), that is, the through-holes (3) plugged at the inflow end face (42) and not plugged at outflow end face (44). In this case, the partition walls (2) act as a filter. For example, soot discharged from a diesel engine is captured by the partition walls and deposited on the partition walls. In the honeycomb filters used in such a condition, there was such a problem that the sharp temperature change of exhaust gas and the local heating made non-uniform the temperature distribution inside the honeycomb structure and caused cracks in a honeycomb filter. When the honeycomb filter is used particularly as a filter for capturing particulate matter in exhaust gas emitted from a diesel engine (this filter is hereinafter referred to as DPF), it is necessary to burn the carbon particles deposited on the filter to remove the particles and regenerate the filter and, in that case, there was such a problem that high temperatures were inevitably developed locally in the filter to deteriorate regeneration efficiency due to non-uniformity of regeneration temperature as well as to cause cracks due to large thermal stress. Further, non-uniform temperature distribution during regeneration made it difficult for the whole portion of the filter to have at an optimum temperature and also made it difficult to have high regeneration efficiency.
Hence, there have been proposed processes for producing a honeycomb filter by bonding a plurality of individual segments using an adhesive. In, for example, U.S. Pat. No. 4335783 is disclosed a process for producing a honeycomb structure, which comprises bonding a large number of honeycomb parts using a discontinuous adhesive. Also in JP-B-61-51240 is proposed a thermal shock resistant rotary regenerative heat exchanger which is formed by extrusion molding matrix segments of honeycomb structure made of a ceramic material; firing them; making smooth, by processing, the outer peripheral portion of the fired segment to form a bonding surface; coating the bonding surface with a ceramic adhesive which turns, after firing, to have substantially the same chemical composition as the matrix segment and a difference in thermal expansion coefficient of 0.1% or less at 800 degree C.; loading the required number of segments; and firing the coated segments. Also in the SAE paper 860008 of 1986 is disclosed a ceramic honeycomb structure obtained by bonding cordierite honeycomb segments with a cordierite cement. Further, in JP-A-8-28246, a ceramic honeycomb structure is disclosed in which a certain number of honeycomb ceramic members are bonded by an elastic seal material including at least a three-dimensionally crossing inorganic fiber, an inorganic binder, an organic binder, and inorganic particles. It was also tried to produce a honeycomb filter using, for example, a silicon carbide based material of high thermal conductivity and high heat resistance, in order to prevent it from localized high temperature to prevent breakage due to thermal stress.
By using segment and/or a highly heat-resistant material such as silicon carbide based material, the damage caused by thermal stress could be prevented to some extent. However, the temperature difference between the outer peripheral portion and center of honeycomb filter could not be eliminated, and improvement in regeneration efficiency by uniform regeneration was insufficient. Further, local heating appeared during regeneration, in some cases.